Mixed powder for powder metallurgy

ABSTRACT

A mixed powder for powder metallurgy according to an embodiment of the present invention contains an iron-based powder as a main component and further contains a powder of at least one sulfide selected from CaS, MnS, and MoS2; and a powder wherein a percentage content of magnesium oxide is greater than or equal to 0.005% by mass and less than or equal to 0.025% by mass, wherein the magnesium oxide has an average particle size D50 of greater than or equal to 0.5 μm and less than or equal to 5.0 μm.

TECHNICAL FIELD

The present invention relates to a mixed powder for powder metallurgy.

BACKGROUND ART

In a powder metallurgy method, for example, a sintered body with acomplex shape such as a net shape or the like can be formed by sinteringan iron-based powder. Such a sintered body is used, for example, as astructural component such as an automobile component or the like. Withan increasing demand for higher dimensional accuracy of such acomponent, the dimensional accuracy needs to be improved by furthercutting the sintered body.

Furthermore, a reduction in production cost of the component is alsostrongly demanded; therefore, cost reduction in a cutting process isalso deemed to be important. In the cutting process, cost can belessened by extending the lifetime of a cutting tool; however, asintered body like that described above tends to have low machinabilityand shorten the lifetime of the cutting tool.

Therefore, it has been common practice to use a mixed powder for powdermetallurgy in which an additive that improves machinability and extendsthe lifetime of the cutting tool is mixed into an iron-based powder.Specifically, as the additive that improves machinability (amachinability improving material), for example, a powder of manganesesulfide (MnS), sulfur (S), or the like is used. Such a machinabilityimproving material serves as a lubricant that reduces resistance incutting or as a starting point for dividing chips, thereby extending thelifetime of the cutting tool.

In general, as a content of the machinability improving material in themixed powder for powder metallurgy is increased, the machinability ofthe sintered body to be formed is improved, and thereby the lifetime ofthe cutting tool is extended. However, when the content of themachinability improving material is increased, a problem arises in whicha mechanical property such as crushing strength or the like of asintered material is degraded or a dimensional change rate is changed bysintering, requiring an additional die. Consequently, in general, thecontent of the machinability improving material in the mixed powder forpowder metallurgy is approximately 0.3% by mass to 0.5% by mass.

Furthermore, demands for cost reduction in the cutting process andimprovement in productivity increase needs for higher cutting speed;however, an effect of the above-described machinability improvingmaterial is relatively low in high-speed cutting. In a case in which0.3% by mass or more of a sulfide is added as the machinabilityimproving material, there arises a problem in which sulfur evaporatesduring sintering, thereby dirtying the sintered body in appearance andpolluting an inside of a sintering furnace, which is likely to damagethe sintering furnace.

For example, Japanese Unexamined Patent Application Publication No.1997-279204 proposes an iron-based mixed powder for powder metallurgycontaining a powder of a CaO-Al₂O₃-SiO₂-based composite oxide at 0.02%by weight to 0.3% by weight. According to the above-described patentdocument, use of a composite oxide containing Ca as a main component canreduce degradation of mechanical properties of a sintered body, preventstaining of the sintered body and damage on a sintering furnace, andreduce abrasion of a cutting tool in high-speed cutting.

However, further increasing demands for cost reduction and improvementin dimensional accuracy of a component require a mixed powder for powdermetallurgy that further excels in machinability.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 1997-279204

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In view of the foregoing circumstances, an object of the presentinvention is to provide a mixed powder for powder metallurgy which canform a sintered material having excellent machinability.

Means for Solving the Problems

A mixed powder for powder metallurgy according to an embodiment of thepresent invention made for solving the above-described problems containsan iron-based powder as a main component and further contains a powderof at least one sulfide selected from CaS, MnS, and MoS₂; and a powderwherein a percentage content of magnesium oxide is greater than or equalto 0.005% by mass and less than or equal to 0.025% by mass, wherein themagnesium oxide has an average particle size D50 of greater than orequal to 0.5 μm and less than or equal to 5.0 μm.

It is conceivable that, in the mixed powder for powder metallurgy, thesulfide serves as a lubricant and generates an oxide which causes amagnesium oxide particle having a relatively small particle size toattach to a surface of a cutting tool, thereby reducing wear of thecutting tool by a hard oxide or the like in a sintered body.Accordingly, a sintered material formed by sintering the mixed powderfor powder metallurgy has excellent machinability and enables thecutting tool to have a relatively long lifetime.

In the mixed powder for powder metallurgy, a total content of thesulfide is preferably greater than or equal to 0.04% by mass and lessthan or equal to 0.20% by mass. This configuration can reducedegradation of mechanical properties and the like of the sinteredmaterial formed by sintering the mixed powder for powder metallurgy.

“Iron-based powder” as referred to herein means a pure iron powder, aniron alloy powder, or a mixed powder thereof, “to contain as a maincomponent” as referred to herein means that a content is greater than orequal to 90% by mass. “Average particle size D50” as referred to hereinmeans a particle size at which an accumulated volume in a particle sizedistribution measured by a laser diffraction scattering method reaches50%.

Effects of the Invention

As described above, the mixed powder for powder metallurgy of thepresent invention can form a sintered material having excellentmachinability.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings as appropriate.

Mixed Powder for Powder Metallurgy

A mixed powder for powder metallurgy according to an embodiment of thepresent invention contains an iron-based powder as a main component andfurther contains a powder of a sulfide and a powder of magnesium oxide(MgO). Furthermore, the mixed powder for powder metallurgy may furthercontain, for example, a copper powder, a graphite powder, a powderlubricant, or the like.

Iron-Based Powder

The iron-based powder, which is the main component of the mixed powderfor powder metallurgy, is not particularly limited; for example, areduced iron-based powder, an atomized iron-based powder, anelectrolytic iron-based powder, or the like can be used.

Furthermore, the iron-based powder is not limited to a pure iron powder;for example, a steel powder obtained by pre-alloying alloy elements (apre-alloyed steel powder), a steel powder obtained by partially alloyingalloy elements (a partially alloyed steel powder), or the like can beused, or a mixture of a plurality of kinds thereof may also be used. Asthe alloy elements, for example, known elements that improve propertiesof a sintered body, such as copper, nickel, chromium, molybdenum,sulfur, and the like, can be contained.

The iron-based powder only needs to be of such a size as to be used as amain raw material powder for powder metallurgy, and the average particlesize D50 of the iron-based powder is not particularly limited; forexample, the average particle size D50 can be greater than or equal to40 μm and less than or equal to 120 μm.

Powder of Sulfide

In the sintered body obtained by sintering the mixed powder for powdermetallurgy, the sulfide remains in a form of original particles. Sincethe sulfide is softer than an iron base, which is a main component ofthe sintered body, machinability of the sintered body is improved; inaddition, the sulfide has lubricity and reduces abrasion in cutting,thereby extending the lifetime of a cutting tool.

Furthermore, in a case of a high cutting speed, the sulfide in thesintered body is desulfurized by heat generated in the cutting,generating an oxide. It is conceivable that the oxide attaches to asurface of the cutting tool and forms a film that protects the cuttingtool, and in addition, the oxide serves as a binder that causes themagnesium oxide, which is very hard, to attach to the surface of thecutting tool.

As the above-described sulfide that can efficiently improvemachinability and enables attachment of the magnesium oxide, at leastone of CaS, MnS, and MoS₂ is used.

The lower limit of a total content of the sulfide is preferably 0.04% bymass, and more preferably 0.06% by mass. Meanwhile, the upper limit ofthe total content of the sulfide is preferably 0.20% by mass, and morepreferably 0.18% by mass. In a case in which the total content of thesulfide is less than the lower limit, machinability may fail to besufficiently improved. Conversely, in a case in which the total contentof the sulfide is greater than the upper limit, mechanical properties ofthe sintered body obtained by sintering the mixed powder for powdermetallurgy may be degraded.

The lower limit of an average particle size D50 of the sulfide such asCaS, MnS, or the like is preferably 1.0 μm, and more preferably 1.5 μm.Meanwhile, the upper limit of the average particle size D50 of thesulfide is preferably 10 μm, and more preferably 8 μm. In a case inwhich the average particle size D50 of the sulfide is less than thelower limit, it may be difficult to uniformly disperse the sulfide inthe mixed powder for powder metallurgy, and/or the mixed powder forpowder metallurgy may become unduly expensive. Conversely, in a case inwhich the average particle size D50 of the sulfide is greater than theupper limit, machinability of the sintered body obtained by sinteringthe mixed powder for powder metallurgy may fail to be sufficientlyimproved.

Powder of Magnesium Oxide

Magnesium oxide is a chemically stable, hard material. For this reason,a powder of the magnesium oxide exists as micro particles even in thesintered body obtained by sintering the mixed powder for powdermetallurgy. The micro particles of the magnesium oxide are attached tothe surface of the cutting tool by the oxide attributed to the sulfide,thereby protecting the cutting tool and improving machinability of thesintered body.

The lower limit of a content of the magnesium oxide is 0.005% by mass,and preferably 0.010% by mass. Meanwhile, the upper limit of the contentof the magnesium oxide is 0.025% by mass, and preferably 0.020% by mass.In a case in which the content of the magnesium oxide is less than thelower limit, wear of the cutting tool may fail to be reduced.Conversely, in a case in which the content of the magnesium oxide isgreater than the upper limit, the dimensional change rate in sinteringmay increase, or mechanical properties such as crushing strength and thelike of the sintered body may be insufficient.

The lower limit of an average particle size D50 of the magnesium oxideis 0.5 μm, and preferably 0.7 μm. Meanwhile, the upper limit of theaverage particle size D50 of the magnesium oxide is 5.0 μm, andpreferably 3.0 μm. In a case in which the average particle size D50 ofthe magnesium oxide is less than the lower limit, an aggregate of theMgO is formed, and it may become more difficult to uniformly dispersethe magnesium oxide in the mixed powder for powder metallurgy. Moreover,in a case in which a weight ratio is constant, the number of MgOparticles becomes large, and the MgO existing at a boundary between ironpowder particles increases, thereby inhibiting sintering. As a result,the dimensional change rate may increase or mechanical properties suchas crushing strength and the like may be insufficient. Meanwhile, in acase in which the average particle size D50 of the magnesium oxide isgreater than the upper limit, sintering may be inhibited, causing adecrease in strength, the cutting tool may be chipped and wear thereofmay be accelerated, the magnesium oxide particles may fail to beattached to the cutting tool, shortening the lifetime of the cuttingtool, and/or processing accuracy may be decreased. In other words, themagnesium oxide having a sufficiently small particle size can attach tothe surface of the cutting tool, extending the lifetime thereof, withoutcausing damage that accelerates wear of the cutting tool.

Copper Powder

The copper powder serves as a binder that bonds particles of theiron-based powder to one another, thereby improving the strength of thesintered body obtained by sintering the mixed powder for powdermetallurgy.

The copper powder can be selected from a wide range of copper powdersthat are used for powder metallurgy; for example, an electrolytic copperpowder, an atomized copper powder, or the like can be used.

The copper powder may be simply mixed into the iron-based powder, may beattached to a surface of the iron-based powder by use of a binder, ormay be mixed into the iron-based powder and subjected to heat treatmentto be attached to the surface of the iron-based powder in a dispersedmanner.

The lower limit of a content of the copper powder depends on strengthand hardness required for the sintered body, and is preferably 0.8% bymass, and more preferably 1.0% by mass. Meanwhile, the upper limit ofthe content of the copper powder is preferably 5.0% by mass, morepreferably 3.0% by mass, and particularly preferably 2.0% by mass. In acase in which the content of the copper powder is less than the lowerlimit, an effect of improving the strength of the sintered body may beinsufficient. Conversely, in a case in which the content of the copperpowder is greater than the upper limit, carbon diffusion may beinhibited and the strength of the sintered body may be insufficient.

The lower limit of an average particle size D50 of the copper powder ispreferably 5 μm, and more preferably 10 μm. Meanwhile, the upper limitof the average particle size D50 of the copper powder is preferably 50μm, and more preferably 40 μm. In a case in which the average particlesize D50 of the copper powder is less than the lower limit, it may bedifficult to uniformly disperse the copper powder in the mixed powderfor powder metallurgy, and/or the mixed powder for powder metallurgy maybecome unduly expensive. Conversely, in a case in which the averageparticle size D50 of the copper powder is greater than the upper limit,the strength of the sintered body obtained by sintering the mixed powderfor powder metallurgy may fail to be sufficiently improved.

Graphite Powder

The graphite powder forms a hard pearlite phase by reacting with iron insintering of the mixed powder for powder metallurgy, thereby increasingthe strength of the sintered body to be obtained.

As the graphite powder, for example, a natural graphite powder, anartificial graphite powder, or the like can be used.

The graphite powder may be simply mixed into the iron-based powder ormay be attached to the surface of the iron-based powder by use of abinder.

The lower limit of a content of the graphite powder is preferably 0.2%by mass, and more preferably 0.5% by mass. Meanwhile, the upper limit ofthe content of the graphite powder is preferably 1.5% by mass, and morepreferably 1.0% by mass. In a case in which the content of the graphitepowder is less than the lower limit, the effect of improving thestrength of the sintered body may be insufficient. Conversely, in a casein which the content of the graphite powder is greater than the upperlimit, toughness of the sintered body may be insufficient.

The lower limit of an average particle size D50 of the graphite powderis preferably 1 μm, and more preferably 3 μm. Meanwhile, the upper limitof the average particle size D50 of the graphite powder is preferably 30μm, and more preferably 20 μm. In a case in which the average particlesize D50 of the graphite powder is less than the lower limit, it may bedifficult to uniformly disperse the graphite powder in the mixed powderfor powder metallurgy, or the mixed powder for powder metallurgy maybecome unduly expensive. Conversely, in a case in which the averageparticle size D50 of the graphite powder is greater than the upperlimit, segregation may occur in the sintered body obtained by sinteringthe mixed powder for powder metallurgy, and the strength of the sinteredbody may fail to be sufficiently improved.

Powder Lubricant

The powder lubricant reduces friction between particles when the mixedpowder for powder metallurgy is compacted, thereby improving formabilitythereof and extending die lifetime. In sintering, the powder lubricantis eliminated through evaporation or thermal decomposition.

As the powder lubricant, for example, a powder of a metal soap such aszinc stearate, of a non-metallic soap such as ethylene bis-amide, or ofthe like is used.

The lower limit of a content of the powder lubricant is preferably 0.2%by mass, and more preferably 0.5% by mass. Meanwhile, the upper limit ofthe content of the powder lubricant is preferably 1.5% by mass, and morepreferably 1.0% by mass. In a case in which the content of the powderlubricant is less than the lower limit, formability of a compact of themixed powder for powder metallurgy may be insufficient. Conversely, in acase in which the content of the powder lubricant is greater than theupper limit, density of the sintered body obtained by sintering themixed powder for powder metallurgy, which has been compacted, may bedecreased and the strength of the sintered body may be insufficient.

The lower limit of an average particle size D50 of the powder lubricantis preferably 3 μm, and more preferably 5 μm. Meanwhile, the upper limitof the average particle size D50 of the powder lubricant is preferably50 μm, and more preferably 30 μm. In a case in which the averageparticle size D50 of the powder lubricant is less than the lower limit,it may be difficult to uniformly disperse the powder lubricant in themixed powder for powder metallurgy, and/or the mixed powder for powdermetallurgy may become unduly expensive. Conversely, in a case in whichthe average particle size D50 of the powder lubricant is greater thanthe upper limit, the strength of the sintered body obtained by sinteringthe mixed powder for powder metallurgy may fail to be sufficientlyimproved.

Advantages

It is conceivable that, in the mixed powder for powder metallurgy, thesulfide serves as a lubricant and generates an oxide which makes amagnesium oxide particle having a small particle size attach to thesurface of the cutting tool, thereby reducing wear of the cutting toolby a hard oxide or the like in the sintered body. Consequently, thesintered material formed by sintering the mixed powder for powdermetallurgy has excellent machinability and enables the cutting tool tohave a relatively lifetime.

Other Embodiments

The above-described embodiment does not limit the configuration of thepresent invention. Therefore, in the above-described embodiment, thecomponents of each part of the above-described embodiment can beomitted, replaced, or added based on the description in the presentspecification and general technical knowledge, and such omission,replacement, or addition should be construed as falling within the scopeof the present invention.

EXAMPLES

Hereinafter, the present invention will be described in detail by way ofExamples; the present invention should not be construed as being limitedto description in the Examples.

Mixed powders for powder metallurgy Nos. 1 to 15 were experimentallyproduced by mixing a copper powder, a graphite powder, a machinabilityimproving material, magnesium oxide, and a powder lubricant into aniron-based powder at their respective proportions shown in Table 1below. It is to be noted that “-” in the table indicates that thematerial is not contained.

It is to be noted that “300M,” an atomized pure iron powder (availablefrom Kobe Steel, Ltd.) having an average particle size D50 of 70 μm, wasused as the iron-based powder. As the copper powder, “CuAtW-250,” awater-atomized copper powder (available from Fukuda Metal Foil & PowderCo., Ltd.) having a sieve opening of 250 μm, was used. As the graphitepowder, “CPB” (available from Nippon Graphite Industries, Co.,Ltd.),having an average particle size D50 of approximately 23 μm, was used. AsCaS or MnS, respectively, calcium sulfide (“CaS” in the table) having anaverage particle size D50 of 4.9 μm, which was obtained by reducingcalcium sulfate (CaSO₄) having an average particle size D50 of 2.4 μm inan atmosphere of a reducing gas such as hydrogen or the like, ormanganese sulfide (“MnS” in the table) having an average particle sizeD50 of 4.9 μm was used. As the magnesium oxide, magnesium oxide havingan average particle size D50 of 0.7 μm, magnesium oxide having anaverage particle size D50 of 2.5 μm, or magnesium oxide having anaverage particle size D50 with 3.2 μm was used. As the powder lubricant,an ethylene bis-amide-based wax having an average particle size D50 of27 μm was used.

TABLE 1 Copper Graphite powder powder Machinability improving materialMagnesium oxide Lubricant Sample Content Content Content Particle sizeContent Content No. [% by mass] [% by mass] Kind [% by mass] D50 [μm] [%by mass] [% by mass] 1 1.5 0.9 CaS 0.04 0.7 0.010 0.8 2 1.5 0.9 CaS 0.042.5 0.020 0.8 3 1.5 0.9 CaS 0.04 3.2 0.020 0.8 4 1.5 0.9 CaS 0.04 6.20.020 0.8 5 1.5 0.9 CaS 0.04 0.7 0.020 0.8 6 1.5 0.9 CaS 0.08 — — 0.8 71.5 0.9 CaS 0.08 0.7 0.010 0.8 8 1.5 0.9 CaS 0.08 0.7 0.020 0.8 9 1.50.9 CaS 0.08 0.7 0.030 0.8 10 1.5 0.9 CaS 0.08 0.7 0.040 0.8 11 1.5 0.9CaS 0.12 0.7 0.020 0.8 12 1.5 0.9 — — 0.7 0.040 0.8 13 1.5 0.9 MnS 0.080.7 0.020 0.8 14 1.5 0.9 — — — — 0.8 15 1.5 0.9 MnS 0.50 — — 0.8

The mixed powders for powder metallurgy Nos. 1 to 15 were each compactedin a die, whereby ring-shaped compacts each having an outer diameter of64 mm, an inner diameter of 24 mm, and a height of 20 mm were formed. Itis to be noted that conditions for compacting were set so that eachcompact had a density of 7.00 g/cm³. The compacts obtained were sinteredin a nitrogen gas atmosphere containing a hydrogen gas at 10% by volumeat a temperature of 1,120° C. for 60 minutes, whereby sintered bodieswere obtained.

Dimensional change rates (green base and die base) in sintering, acrushing strength, and a Rockwell hardness (B scale) of each sample weremeasured. Furthermore, a turning test was performed in which a sidesurface of a stack of ten sintered bodies of each sample was processedby turning. As a cutting tool, “SNMN120408,” a chip using “NX2525,” acermet (both available from Mitsubishi Materials Corporation), was used.The side surface was cut by 5,287 m using dry cutting under cuttingconditions in which a peripheral velocity was 200 m/min, a cutting depthwas 0.15 mm/pass, and a feed amount was 0.08 mm/rev.

A parallel wear width (flank wear width Vb) of a flank face of thecutting tool after the turning test was measured.

A surface roughness Ra (arithmetic mean roughness) and a surfaceroughness Rz (maximum height) of a cutting surface of the sinteredbodies after the turning test were measured. These surface roughnessvalues were measured at three points respectively by “SJ-410,” a surfaceroughness meter (available from Mitutoyo Corporation), wherein cutoffvalues were λc=0.8 mm and λs=2.5 μm and a measurement length was 5.0 mm,and an average value of measured values at the three points wascalculated.

Measured values of each of the above-described parameters are summarizedin Table 2 below.

TABLE 2 Surface Surface Green Dimensional change rate Sintered CrushingFlank wear roughness roughness Sample density Green base Die basedensity Rockwell strength width Ra Rz No. [g/cm³] [%] [%] [g/cm³]hardness [MPa] [μm] [μm] [μm] 1 7.00 7.00 0.35 6.96 77.1 870 62.2 0.904.56 2 7.00 7.00 0.36 6.94 77.6 870 65.9 1.09 5.19 3 7.00 7.00 0.36 6.9377.8 868 82.2 1.02 4.88 4 7.00 7.00 0.34 6.95 78.3 875 173.2 1.62 8.16 56.99 6.99 0.36 6.94 77.2 856 57.3 1.32 6.12 6 7.00 7.00 0.35 6.94 78.0857 150.3 1.29 5.86 7 7.00 7.00 0.36 6.93 77.1 851 118.3 0.86 4.37 87.00 7.00 0.37 6.93 77.2 850 105.6 1.17 5.43 9 6.99 6.99 0.38 6.93 77.4834 94.6 0.99 4.38 10 7.01 7.01 0.41 6.93 76.7 822 89.5 0.70 4.25 116.99 6.99 0.38 6.93 77.2 839 104.8 1.00 4.09 12 7.00 7.00 0.37 6.94 77.4857 299.1 2.23 10.37 13 7.01 7.01 0.34 6.96 79.1 867 68.6 0.33 1.97 147.00 7.00 0.33 6.97 78.9 879 250.6 1.35 6.79 15 6.99 6.99 0.37 6.95 78.0840 144.6 0.82 3.76

The sintered bodies obtained by sintering the mixed powders for powdermetallurgy Nos. 1 to 3, 5, 7, 8, 11, and 13, each of which contained asulfide and magnesium oxide and in which a percentage content of themagnesium oxide was greater than or equal to 0.010% by mass and lessthan or equal to 0.020% by mass and an average particle size D50 of themagnesium oxide was greater than or equal to 0.5 μm and less than orequal to 5.0 μm, showed sufficient formability and sufficient mechanicalstrength, and caused little wear of the cutting tool.

Moreover, a drilling test was performed on the sintered bodies formedfrom the mixed powders for powder metallurgy Nos. 1, 5, 8, 13, 14, and15. As a drill, “AD-4D,” a coated carbide drill (available from OSGCorporation) having a diameter of 3.8 mm, was used. Processingconditions were as follows: a peripheral velocity of the drill was 2m/min (4,358 rpm), a feed rate was 450 mm/min (0.103 mm/rev), and“Yushiroken EC50,” a water-soluble fluid (available from YUSHIROCHEMICAL INDUSTRY CO.,LTD.), was poured as a cutting oil onto thesintered bodies during cutting. To ensure a cutting distance, 180non-through holes were formed with depths of 10 mm.

In the drilling test, a flank wear width (Vb) of the drill was measuredupon forming every thirtieth non-through hole. Table 3 below showsmeasurement results.

TABLE 3 Sample Cutting distance [mm] No. 0 300 600 900 1200 1500 1800 10 43 60 66 71 73 77 5 0 39 50 54 57 60 65 8 0 37 61 64 70 79 84 13 0 4860 67 70 74 79 14 0 44 68 72 77 82 92 15 0 49 63 72 78 82 88 Flank wearwidth [μm]

The sintered bodies obtained by sintering the mixed powders for powdermetallurgy Nos. 1, 5, 8, and 13, each of which contained the sulfide andthe magnesium oxide, caused less wear of the cutting tool than that ofthe sintered body obtained by sintering the mixed powder for powdermetallurgy No. 14, which contained neither the sulfide nor the magnesiumoxide, and the sintered body obtained by sintering the mixed powder forpowder metallurgy No. 15, which contained the sulfide but did notcontain the magnesium oxide.

Mixed powders for powder metallurgy Nos. 16 to 32 were experimentallyproduced by mixing a copper powder, a graphite powder, a machinabilityimproving material, magnesium oxide, and a powder lubricant into aniron-based powder at their respective proportions shown in Table 4below. It is to be noted that “-” in the table indicates that thematerial is not contained.

It is to be noted that the iron-based powder, the copper powder, thegraphite powder, the magnesium oxide, and the powder lubricant were ofthe same types and proportions those used for the mixed powders forpowder metallurgy Nos. 1 to 15. As the machinability improving material,manganese sulfide of the same type and proportion to that in the mixedpowders for powder metallurgy Nos. 1 to 15, sulfur (“S” in the table)having an average particle size D50 of 46.1 μm, and iron sulfide (“FeS”in the table) having an average particle size D50 of 13.5 μm, the sulfurand the iron sulfide having passed a 100-mesh wire screen, were used.

TABLE 4 Copper Graphite powder powder Machinability improving materialMagnesium oxide Lubricant Sample Content Content Content Particle sizeContent Content No. [% by mass] [% by mass] Kind [% by mass] D50 [μm] [%by mass] [% by mass] 16 2.0 0.8 MnS 0.04 0.7 0.010 0.8 17 2.0 0.8 MnS0.06 0.7 0.010 0.8 18 2.0 0.8 MnS 0.08 0.7 0.010 0.8 19 2.0 0.8 MnS 0.080.7 0.020 0.8 20 2.0 0.8 MnS 0.08 0.7 0.030 0.8 21 2.0 0.8 MnS 0.08 0.70.040 0.8 22 2.0 0.8 MnS 0.10 0.7 0.010 0.8 23 2.0 0.8 MnS 0.12 0.70.010 0.8 24 2.0 0.8 MnS 0.08 2.5 0.020 0.8 25 2.0 0.8 MnS 0.08 3.20.020 0.8 26 2.0 0.8 MnS 0.08 6.2 0.020 0.8 27 2.0 0.8 MnS 0.50 — — 0.828 2.0 0.8 S 0.08 46.1  0.010 0.8 29 2.0 0.8 S 0.50 — — 0.8 30 2.0 0.8FeS 0.08 13.5  0.010 0.8 31 2.0 0.8 FeS 0.50 — — 0.8 32 2.0 0.8 — — — —0.8

The mixed powders for powder metallurgy Nos. 16 to 32 were eachcompacted in a die in a manner similar to that of the mixed powders forpowder metallurgy Nos. 1 to 15, whereby ring-shaped compacts wereformed; the compacts obtained were sintered in a nitrogen gas atmospherecontaining a hydrogen gas at 10% by volume at a temperature of 1,130° C.for 60 minutes, whereby sintered bodies were obtained.

Dimensional change rates (green base and die base) in sintering, acrushing strength, and a Rockwell hardness (B scale) of each sample weremeasured. Furthermore, a turning test was performed in which a sidesurface of a stack of ten sintered bodies of each sample was processedby turning. As a cutting tool, “2NU-CNGA120408LF,” a chip using“BN7500,” cubic boron nitride, (both available from Sumitomo ElectricIndustries, Ltd.), was used. The side surface was cut by 2,735 m by drycutting under cutting conditions in which a peripheral velocity was 200m/min, a cutting depth was 0.1 mm/pass, and a feed amount was 0.1mm/rev.

A parallel wear width (flank wear width Vb) of a flank face of thecutting tool after the turning test was measured.

Measured values of each of the above-described parameters are summarizedin Table 5 below.

TABLE 5 Surface Surface Green Dimensional change rate Sintered CrushingFlank wear roughness roughness Sample density Green base Die basedensity Rockwell strength width Ra Rz No. [g/cm³] [%] [%] [g/cm³]hardness [MPa] [μm] [μm] [μm] 16 7.01 0.13 0.45 6.92 73.7 834 35.6 0.804.65 17 7.01 0.14 0.45 6.92 73.2 829 25.4 0.64 3.85 18 7.01 0.14 0.466.92 73.4 827 19.3 0.70 5.00 19 7.00 0.16 0.48 6.90 73.3 820 19.1 0.626.05 20 7.00 0.19 0.50 6.90 73.6 794 20.7 0.68 6.53 21 7.00 0.22 0.536.89 74.0 789 29.1 0.75 7.71 22 7.01 0.16 0.47 6.92 74.2 827 23.1 0.755.10 23 7.01 0.16 0.47 6.91 73.8 824 21.6 0.69 6.07 24 7.00 0.16 0.486.90 73.6 824 24.7 0.71 6.17 25 7.00 0.17 0.49 6.89 73.3 822 36.2 0.736.25 26 7.00 0.15 0.47 6.91 74.6 844 121.7 0.73 6.25 27 7.00 0.19 0.506.91 73.9 799 138.7 0.45 2.38 28 7.01 0.44 0.75 6.85 71.4 708 42.1 0.686.28 29 6.99 0.68 1.02 6.75 68.8 715 88.0 0.32 3.42 30 7.00 0.22 0.536.90 72.6 787 38.3 0.77 4.79 31 7.01 0.52 0.85 6.93 68.2 753 182.7 0.733.30 32 7.00 0.12 0.43 6.93 74.4 853 352.9 0.39 2.04

The sintered bodies obtained by sintering the mixed powders for powdermetallurgy Nos. 16 to 19 and 22 to 25, each of which contained sulfidesand magnesium oxide and in which a percentage content of the magnesiumoxide was greater than or equal to 0.010% by mass and less than or equalto 0.020% by mass and an average particle size D50 of the magnesiumoxide was greater than or equal to 0.5 μm and less than or equal to 5.0μm, showed sufficient formability and sufficient mechanical strength,and caused little wear of the cutting tool.

Mixed powders for powder metallurgy Nos. 33 to 39 were experimentallyproduced by mixing a copper powder, a graphite powder, a machinabilityimproving material, magnesium oxide, and a powder lubricant into aniron-based powder at their respective proportions shown in Table 6below. It is to be noted that “-” in the table indicates that thematerial is not contained.

It is to be noted that the iron-based powder, the copper powder, thegraphite powder, the magnesium oxide, and the powder lubricant were ofthe same types and proportions to those used for the mixed powders forpowder metallurgy Nos. 1 to 15. As the machinability improving material,manganese sulfide of the same type and proportion to that in the mixedpowders for powder metallurgy Nos. 1 to 15 and molybdenum disulfide(“MoS₂” in the table) having an average particle size D50 of 5.0 μm wereused.

TABLE 6 Copper Graphite Machinability improving Machinability improvingpowder powder material 1 material 2 Magnesium oxide Lubricant SampleContent Content Content Content Particle size Content Content No. [% bymass] [% by mass] Kind [% by mass] Kind [% by mass] D50 [μm] [% by mass][% by mass] 33 2.0 0.8 MnS 0.08 — — 0.7 0.010 0.8 34 2.0 0.8 — — MoS₂0.08 0.7 0.010 0.8 35 2.0 0.8 MnS 0.04 MoS₂ 0.02 0.7 0.010 0.8 36 2.00.8 MnS 0.04 MoS₂ 0.04 0.7 0.010 0.8 37 2.0 0.8 MnS 0.06 MoS₂ 0.04 0.70.010 0.8 38 2.0 0.8 — — MoS₂ 0.50 — — 0.8 39 2.0 0.8 — — — — — — 0.8

The mixed powders for powder metallurgy Nos. 33 to 39 were eachcompacted in a die in a manner similar to that of the mixed powders forpowder metallurgy Nos. 16 to 32, whereby ring-shaped compacts wereformed; the compacts obtained were sintered under conditions similar tothose of Nos. 16 to 32, whereby sintered bodies were obtained.

Dimensional change rates (green base and die base) in sintering, acrushing strength, and a Rockwell hardness (B scale) of each sample weremeasured. Furthermore, a turning test was performed in which a sidesurface of a stack of ten sintered bodies of each sample was processedby turning. As a cutting tool, “2NU-CNGA120408LF,” a chip using“BN7500,” cubic boron nitride, (both available from Sumitomo ElectricIndustries, Ltd.), was used. The side surface was cut by 2,735 m by drycutting under cutting conditions in which a peripheral velocity was 200m/min, a cutting depth was 0.1 mm/pass, and a feed amount was 0.1mm/rev.

Parallel wear width (flank wear width Vb) of a flank face of the cuttingtool after the turning test was measured.

Measured values of each of the above-described parameters are summarizedin Table 7 below.

TABLE 7 Surface Surface Green Dimensional change rate Sintered CrushingFlank wear roughness roughness Sample density Green base Die basedensity Rockwell strength width Ra Rz No. [g/cm³] [%] [%] [g/cm³]hardness [MPa] [μm] [μm] [μm] 33 7.01 0.20 0.51 6.92 74.7 814 30.5 0.775.79 34 7.00 0.25 0.56 6.90 75.6 795 28.1 0.73 5.50 35 7.00 0.20 0.516.91 75.8 810 26.3 0.74 5.87 36 7.00 0.22 0.53 6.91 76.3 790 24.0 0.786.28 37 7.00 0.23 0.54 6.90 76.0 801 25.4 0.80 6.33 38 7.00 0.36 0.696.86 79.7 908 78.1 1.12 5.07 39 7.01 0.14 0.46 6.93 77.4 853 184.3 0.573.65

The sintered bodies obtained by sintering the mixed powders for powdermetallurgy Nos. 33 to 37, each of which contained a sulfide andmagnesium oxide and in which a percentage content of the magnesium oxidewas greater than or equal to 0.010% by mass and less than or equal to0.020% by mass, showed sufficient formability and sufficient mechanicalstrength, and caused little wear of the cutting tool. Furthermore, incomparison between No. 33 and No. 34, No. 34, in which the molybdenumdisulfide was used as the sulfide, caused a smaller parallel wear widthof the flank face than that of No. 33, in which the manganese sulfidewas used as the sulfide. Moreover, Nos. 35 to 37, in each of which twokinds of sulfides (the manganese sulfide and the molybdenum disulfide)were used, caused smaller parallel wear widths of the flank faces thanthose of No. 33 and No. 34, in each of which only one of the manganesesulfide and the molybdenum disulfide was used as the sulfide. It is tobe noted that, although No. 33 is identical to No. 18 in proportions ofthe components and No. 39 is identical to No. 32 in proportions of thecomponents, it is conceivable that differences in the measured valueswere caused by a difference in batch of each material.

INDUSTRIAL APPLICABILITY

The mixed powder for powder metallurgy according to the presentinvention is suitably used for producing a high-precision componentwhich requires cutting after sintering.

1. A mixed powder suitable for powder metallurgy, the powder comprising:an iron-comprising powder as a main component; a first powder comprisingCaS, MnS, and/or MoS₂, as a sulfide; and a second powder havingpercentage content of magnesium oxide in a range of from 0.005 to 0.025mass %, based on total mixed powder weight, wherein the magnesium oxidehas an average particle size D50 in a range of from 0.5 to 5.0 μm. 2.The powder of claim 1, wherein the sulfide is present in a range of from0.04 to 0.20 mass %.
 3. The powder of claim 1, wherein the first powdercomprises CaS.
 4. The powder of claim 1, wherein the first powdercomprises MnS.
 5. The powder of claim 1, wherein the first powdercomprises CaS and MnS.
 6. The powder of claim 1, wherein the firstpowder comprises MoS₂.
 7. The powder of claim 1, wherein the firstpowder comprises CaS and MOS₂.
 8. The powder of claim 1, wherein thefirst powder comprises MnS and MoS7.
 9. The powder of claim 1, whereinthe first powder comprises CaS, MnS, and MoS₂.
 10. The powder of claim1, wherein the magnesium oxide has an average particle size D50 in arange of from 0.7 to 3.0 μm.
 11. The powder of claim 1, furthercomprising: copper in an amount of from 0.8 to 3.0 mass %, based on thetotal mixed powder weight.
 12. The powder of claim 1, wherein theiron-comprising powder is present in an amount of at least 95.86 mass %,based on the total mixed powder weight.
 13. The powder of claim 1,comprising the iron in an amount of at least 95.86 mass %, based on thetotal mixed powder weight.
 14. The powder of claim 1, wherein theiron-comprising powder is non-alloyed steel.
 15. The powder of claim 1,wherein the iron-comprising powder is at least partially alloyed steel.16. The powder of claim 1, wherein the iron-comprising powder furthercomprises copper, nickel, chromium, and/or molybdenum.
 17. The powder ofclaim 2, wherein the sulfide is present in at least 0.06 mass %.
 18. Thepowder of claim 2, wherein the sulfide is present in at most 0.18 mass%.
 19. The powder of claim 1, wherein the sulfide has an averageparticle size D50 in a range of from 1.0 to 10 μm.